In recent years, lithium secondary batteries have been widely used as driving power supplies for mobile electronic devices and communication devices. In such a lithium secondary battery, in general, a carbon material capable of inserting and extracting lithium is used for a negative electrode plate and a composite oxide, such as LiCoO2 or the like, containing transition metal and lithium is used for a positive electrode plate, thereby achieving a secondary battery with a high potential and a high discharge capacity. Now, with the development of electronic devices and communication devices having an increased range of functions, further increase in capacity is desired.
To realize a high capacity lithium secondary battery, for example, a volume of a positive electrode plate and a negative electrode plate occupying in a battery case is increased and an empty space other than a space between the electrode plates in the battery case is reduced. Thus, the capacity of the lithium battery can be further increased.
Moreover, for mixture pastes for positive and negative electrode plates, a mixture paste made of a material of a positive electrode plate or a negative electrode plate is applied onto a current collector core material and dried to form an active material layer and then, a high pressure is applied the active material by roll pressing to compress the active material to a predetermined thickness, thereby increasing a filling density. Thus, a further increase in capacity can be achieved.
When a filling density of an active material in each electrode plate is increased, it becomes difficult to impregnate a nonaqueous electrolyte with a relatively high viscosity, which has been injected into a battery case, into small gaps in an electrode group formed of positive and negative electrode plates stacked or spirally wound with a separator interposed therebetween with a high density. Therefore, it requires a long time to impregnate a predetermined amount of the nonaqueous electrolyte into the electrode group. Furthermore, with an increased filling density of the active material of each electrode plate, a porosity in each electrode plate is reduced and it is more difficult to impregnate the electrolyte thereinto. Therefore, the ability of impregnating the electrode group with the nonaqueous electrolyte is further reduced and, as a result, the distribution of the nonaqueous electrolyte in the electrode group becomes nonuniform.
In Patent Document 1, a method in which electrolyte guiding grooves are formed in a surface of a negative electrode active material layer along a penetrating direction of the nonaqueous electrolyte to achieve proper impregnation of the nonaqueous electrolyte into an entire negative electrode is described. It should be noted that an impregnation time can be reduced by increasing width and depth of the grooves but, by doing so, an amount of the active material is reduced. This causes reduction in charge/discharge capacity and nonuniform reactions between electrode plates, so that a battery property is deteriorated. Therefore, taking this into consideration, the width and depth of the grooves are set to be predetermined values.
The grooves formed in the surface of the negative electrode active material layer might cause fractures of electrode plates when the electrode plates are wound to form an electrode group. Patent Document 2 discloses a method for improving impregnation while preventing fractures of electrode plates.
Specifically, grooves are formed in surfaces of electrode plates so that each of the grooves makes an angle with respect to a longitudinal direction of the electrode plate, thereby distributing tensile force applied in the longitudinal direction when the electrode plate is wound to form an electrode group. Thus, fractures of the electrode plates can be prevented.
In Patent Document 3, a method in which a porous film having convex portions partially formed on a surface thereof facing a positive electrode or a negative electrode is provided, not for the purpose of improving impregnation with an electrolyte but for the purpose of suppressing overheat caused by overcharge, is described. That is, a larger amount of a nonaqueous electrolyte is maintained in spaces formed between the convex portions of the porous film and an electrode plate than in other spaces to induce overcharge reaction in the spaces in a concentrated manner. By doing so, overcharge of a battery as a whole can be suppressed and overheat due to overcharge can be suppressed.
In a lithium secondary battery of which a capacity is increased by the above-described means, for example, when a foreign material is mixed in the battery for some reason and causes damage of a separator and thus a short circuit between a positive electrode plate and a negative electrode plate occurs, a current flow concentrates in short circuited part and rapidly generates heat. This might cause decomposition of a positive electrode material and a negative electrode material, the generation of gas due to boiling or decomposition of an electrolyte, and the like.
To cope with problems resulting from the internal short circuit, methods in which a porous protective film is formed to coat a surface of a negative electrode active later or a positive electrode active material layer and thereby the generation of an internal short circuit is suppressed are described in Patent Documents 4 and 5 and the like.
Patent Document 1: Japanese Published Patent Application No. 9-298057
Patent Document 2: Japanese Published Patent Application No. 11-154508
Patent Document 3: Japanese Published Patent Application No. 2006-12788
Patent Document 4: Japanese Published Patent Application No. H7-220759
Patent Document 5: International Publication No. 2005/029614 (pamphlet)